1. Field of the Invention
The present invention relates to a non-aqueous ink that is suitable for use with an inkjet recording system, and more particularly to a non-aqueous inkjet ink that is suitable for low-resolution printing.
2. Description of the Related Art
Inkjet recording systems eject a highly fluid inkjet ink from very thin head nozzles as ink particles to record an image on a sheet of printing paper, or the like, which is positioned to face the nozzles. Because of low noise and ability of high-speed printing, the inkjet recording systems are rapidly becoming widely used in recent years. Inkjet inks used with inkjet printing systems are generally classified into aqueous inks and non-aqueous inks. The non-aqueous inks are characterized by having good on-machine stability, such as good intermittent ejection property and ejection recovery after being left for a long time, not causing curl of printing paper, and penetrating and drying in a short time, and thus are widely used. For example, the applicant of the present application has proposed, in U.S. Pat. No. 7,799,123 (hereinafter, Patent Document 1), a non-aqueous ink including a pigment and an organic solvent, such as an ester solvent, a higher alcohol solvent, a hydrocarbon solvent, etc., and further including a soluble polymeric dispersant.
On the other hand, with respect to printing systems, there are demands for low-resolution systems in view of increasing printing speed and reducing hardware cost. Pigment inks, such as one disclosed in Patent Document 1, are characterized by that they do not easily spread, since the colorant thereof has a large particle size and is caught up in the fibers of the printing paper while the ink penetrates into the printing paper and is fixed, and that they provide a highly durable and water resistant image. However, the advantage of the pigment inks that they do not easily spread becomes a problem when the pigment inks are used for low-resolution printing, where a distance between pixels is large and the pigment inks fail to fill gaps between the pixels, resulting in a low image density.
When dye inks using a dye, such as one disclosed in Japanese Unexamined Patent Publication No. S63 (1988)-113089 (hereinafter, Patent Document 2), are used, the dye moves along with penetration of the solvent and the resulting image is blurred. Therefore, when the dye inks are used for low-resolution printing, they fill gaps between the pixels. However, the dye itself moves deep into the printing paper along with the penetration of the ink, resulting in a very low surface density and a high degree of strike-through. On the other hand, a pigment-dye combined ink using a pigment and a dye in combination (which may hereinafter be referred to as “combined ink”) is disclosed, for example, in Japanese Unexamined Patent Publication No. H07 (1995)-109430 (hereinafter, Patent Document 3).
It may be considered that, when a pigment and a dye are used in combination for low-resolution printing, the dye moves to fill gaps between the pixels to thereby provide a high image density. However, when the combined ink having the composition as disclosed in Patent Document 3 is used for low-resolution printing, the pigment stays at the center of each dot and a blurred color is formed by the spread dye around the center, resulting in an image having very poor image quality. Therefore, it is difficult to provide an image having an appropriate dot gain and a desired image density by low-resolution printing using the conventional combined inks.